1. Field of the Invention
The present invention relates to passive optical networks and in particular to a novel host receiver synchronizer for passive optical networks.
2. Description of the Prior Art
Passive optical networks (PONs) are optical networks in which there is a shared host transceiver and a plurality of subscriber nodes. Each of the subscriber nodes are connected to the host transceiver by a single shared optical fiber through optical splitters.
Transmission in the downstream direction, that is, from the host transmitter to each subscriber node is accomplished by broadcasting a signal on a designated wavelength from the host transmitter through a series of optical splitters to each of the subscriber nodes. The data stream in the downstream direction contains messages which may be intended for any of the subscribers and each subscriber reads the downstream data. The data stream is encoded to identify to which subscriber a portion thereof is intended and that subscriber recognizes when it is the intended recipient of the data and extracts and processes it.
Transmission from the subscriber nodes back to the host (in the “uplink” direction) is done through the same optical splitter network and fiber, but on a second optical wavelength. The uplink bandwidth is shared between subscribers using time division multiplexing (TDM). Accordingly, the uplink data stream is necessarily bursty, as subscribers only transmit when they have data to send and at specific times to avoid collisions with other subscribers.
Because a single host optical transceiver is shared by many subscribers, PONs, including networks complying with the EPON (Ethernet PON) or GPON (Gigabit PON) standards are a relatively low cost solution while providing substantially higher data rates than the present state of the art for either wired or wireless access solutions.
In the more aggressive PON standard, GPON, maximum permissible data rates are 2.48832 Gbit/s in both the downstream and uplink directions, though in practice only 1.24416 Gbit/s is presently feasible for the uplink direction. Discussion is presently underway for future systems with 9.95328 Gbit/s and beyond.
Moreover, PON systems provide substantial economy over direct dedicated point to point solutions such as Ethernet or SONET. Accordingly, PONS are rapidly becoming the emerging access solution for the delivery of broadband services to the business and residential market.
Under the PON standard specifications, in the uplink direction, subscribers transmit their data in bursts at a frequency synchronized to the downstream data frequency. The subscribers recover the transmit clock signal from the downstream data and use this clock to retime their upstream data back to the host making the entire system frequency synchronous.
Even though all subscriber transmitters are synchronous with the host, they are all at different distances from the host and may also have different latencies before transmission occurs so the phase of the upstream data arriving at the host will vary widely from burst to burst.
Since the phase of every burst may be different, the phase of the host receiver sampling clock must be realigned with every burst of upstream data. Until the phase of the sampling clock has been successfully realigned to the data, any data received will be lost. To avoid loss of usable bandwidth it is preferable to minimize the portion of the burst that is lost during phase realignment.
Because of the bursty nature of upstream transmissions, conventional feedback mechanisms for clock recovery and resynchronization are not generally applicable as they typically take considerable time to approach the received frequency and then match its phase. Rather, any clock resynchronization mechanism must adapt to each burst in turn in a time much shorter than the burst duration.
In the various PON standards, each burst is preceded by a preambular period of short duration relative to the burst length in which a pattern providing expendible data transitions is transmitted for the purpose of allowing phase alignment of the receiver sampling clock to the burst. The use of a tone in particular ensures that there are a large number of rising and falling transitions to aid in the resynchronization process.
In GPON systems in particular, phase alignment should occur within the preambular period since a delimiter follows the preamble supplying framing information for the remainder of the burst. If the delimiter is incorrectly received, then the burst will be lost.
Conventional synchronizers for optical systems may require on the order of hundreds of microseconds to resynchronize to a data stream after a prolonged period without data applied to the input. However, the brevity of the preambular period mandates that resynchronization be completed in substantially less than 100 nanoseconds.
In current PON systems, digital oversampling techniques are applied to choose the optimal sampling point from among many samples per data bit period, using a digital state machine.
The granularity of sampling limits the accuracy of phase alignment. Improved granularity of sampling costs power and chip area and can be limited by matching concerns.
Accordingly, such prior art methodologies are only appropriate for relatively low uplink data rates, presently 1.24432 Gbit/s and below, but would not be feasible using currently available technology at rates of 2.48832 Gbit/s or above. Even at the preferred 1.24416 Gbit/s uplink data rate for GPON, the receiver performance would be limited by the oversample resolution.
This is because oversampling techniques suffer numerous limitations such as bandwidth, noise, and matching, all of which degrade the ability of the resynchronization circuit to synchronize at an optimal sampling position. This makes PON systems embodying such techniques very sensitive to signal degradation and may even result in loss of data when using longer fiber lengths.
Linear clock and data recovery (CDR) circuits, that is, a CDR circuit that continuously adjusts the phase of the recovered clock with infinite precision adapting to the centre of the data eye, have been conventionally used to adjust the frequency and phase of a local clock to optimally sample the data, using a feedback loop that compares the period of the received data to the period of a local clock and continuously adjusts the phase and period of the local clock to maintain a proper sample point even as the phase of the data changes. For the purposes of the discussion below, such maintaining of a proper sample point is referred to as frequency and/or phase locking to the local clock.
However, linear CDR circuits first match the frequency of the incoming data in an initial frequency lock state and it is only when frequency lock is achieved that the linear CDR circuits deal with the phase of the incoming data in a subsequent phase lock state. Unfortunately, typically, the length of the frequency lock state is, in such conventional systems, much longer than the time available for locking to a PON burst. Thus, such conventional systems are unsuited to burst mode applications.
Accordingly, it is desirable to provide a novel resynchronizer for resynchronizing the host receiver sampling clock in a PON system.
It is further desirable to provide a resynchronizing circuit that will automatically centre in the middle of the data eye without oversampling.
It is still further desirable to provide a resynchronizing circuit that is capable of operating at a present preferred 1.24416 Gbit/s uplink data rate and future 2.48832 Gbit/s rate as well higher rates that may be specified in the future.